What is Form CT1 (Corporation Tax Return)?

Form CT1 is Ireland's annual Corporation Tax return, filed exclusively through ROS (Revenue Online Service). It covers the company's accounting period and calculates the Corporation Tax liability at 12.5% on trading income, 25% on non-trading income, and 33% on chargeable gains.

**When the CT1 is due**

For ROS filers, the CT1 must be filed and the balance of tax paid by the 23rd of the ninth month after the accounting period ends. For paper filers, the deadline is the 21st. Examples:

- Year ending 31 December 2024: CT1 due 23 September 2025 - Year ending 31 March 2025: CT1 due 23 December 2025 - Year ending 30 June 2025: CT1 due 23 March 2026 - Year ending 30 September 2025: CT1 due 23 June 2026

Note that a 31 December year-end triggers a September CT1 deadline — not the intuitive "end of the year" that many directors expect. Companies frequently miss this because the September date feels early.

**Preliminary tax — the payment that comes before year-end**

Ireland's preliminary tax system requires companies to pay estimated tax before the year closes, not after. This is the most confusing aspect of Irish corporate tax compliance for directors used to the UK system.

For small companies (prior year CT liability under €200,000): preliminary tax of at least 100% of the prior year's CT liability is due on the 23rd of the month before the accounting period ends: - December year-end: preliminary tax due 23 November of the same year - March year-end: preliminary tax due 23 February of the same year - June year-end: preliminary tax due 23 May of the same year

Alternatively, a company can pay 90% of the estimated current year liability instead — useful if profits have dropped significantly.

For large companies (prior year CT over €200,000): two instalments are required: 1. First instalment: 45% of prior year CT (or 50% of current year estimate) — due in the 6th month of the accounting period 2. Second instalment: balance to bring total preliminary tax to 90% of current year — due in the 11th month of the accounting period

**What the CT1 contains**

The CT1 is a detailed return covering: - Trading income calculation: accounting profit adjusted for disallowable expenses (client entertainment, depreciation replaced by capital allowances, etc.) and stock relief - Capital allowances: wear and tear allowances on plant, machinery, and equipment (12.5% per year straight-line under the standard Irish rules) - Non-trading income: rental income, investment income, certain dividends from foreign subsidiaries - Chargeable gains: disposals of capital assets during the period (33% rate, but calculated separately from trading income) - R&D tax credit: 30% credit on qualifying research and development expenditure (claimable as a cash refund or offset against CT) - Group relief: losses surrendered from other group companies within an Irish Corporation Tax group - Close company surcharge: if the company is a close company and has undistributed investment or estate income above a threshold, a 20% surcharge applies. This is separate from, but conceptually similar to, the UK S455 charge on loans to participators (though the Irish mechanism is different) - Transfer pricing disclosure: required for large companies with cross-border related-party transactions above €250,000 per category

**How to file**

The CT1 is prepared by the company's accountant and submitted via the accountant's ROS agent login. The return is pre-populated with company registration details. Revenue does not send a reminder — it is the company's responsibility to know the filing date.

Penalties for late filing: - Up to 2 months late: 5% surcharge on the CT due, capped at €12,695 - More than 2 months late: 10% surcharge, capped at €63,485 - Flat €500 fine for failure to file within the period

**Small company vs large company practical differences**

The vast majority of Irish SMEs qualify as small companies (prior CT under €200,000). The main practical differences are: - Single preliminary tax instalment vs two for large companies - No transfer pricing documentation obligation below the large company threshold - Different rules for group relief between small and large group members

Pillar Two rules (effective from 1 January 2024) impose a minimum 15% effective tax rate on multinational groups with consolidated revenue over €750 million. These companies file supplementary Pillar Two returns alongside the standard CT1.